In general, a synthetic resin molded article which requires flame retardancy is blended with any of various flame retardants such as a phosphorus based flame retardant and a halogen based flame retardant. These flame retardants are required to be resistant to the heat, acting in the case of molding and processing a synthetic resin or using a product after molding and at the same time, are desired not to impair the performances inherent in the synthetic resin such as water resistivity and physical performances.
However, the phosphorus-containing compounds which have hitherto been used as a flame retardant suffer from such defects that impair physical characteristics of a synthetic resin, deteriorate the stability and water resistivity and so forth, since most of the compounds have been each an additive-type flame retardant. In addition, since most of the phosphorus-containing compounds that are used each as an additive-type flame retardant are poor in compatibility with a resin, there have been brought about the problem that the compounds are difficult to homogeneously blend in the resin, even if homogeneous blending is possible, the problem that the flame retardant bleeds out from the molded article after molding, thus failing to maintain the working effect thereof and also the problem on external appearance.
In order to overcome the above-mentioned problems, a phosphorus-containing epoxy resin is proposed {refer to Japanese Patent Application Laid-Open No. 279258/1999 (Hei 11)}. The aforesaid epoxy resin, in which phosphorus-containing groups are chemically bonded to the resin matrix after molding, can become a flame retardant excellent in water resistivity and stability, while maintaining resin characteristics to some extent. Nevertheless, the phosphorus-containing epoxy resin is inferior in solubility in much of organic solvents and in compatibility with a variety of resins, thereby limiting the blending formulations thereof. It also involves the problem that it is inapplicable alone to the purpose of the use which requires high flame retardancy because of a low concentration (2.5% by weight or lower) of phosphorus atoms that can be blended in a resin and the like.
On the other hand, although some consideration is given to a phosphorus-containing carboxylic acid having phosphorus atoms and further a carboxylic group which reacts with an epoxy group as an excellent reactive flame retardant, the carboxylic acid involves the problem about general purpose properties because of its being inferior in solubility in much of organic solvents and in compatibility with a variety of resins as mentioned hereinbefore. In addition, since the carboxylic group and a reactive group are prone to react with each other, the carboxylic acid further creates the problem about the stability in that a composition in which the phosphorus-containing carboxylic acid and a compound containing the aforesaid reactive group coexist causes gelation during storage, limits a usable hour from blending to application and so forth.
Moreover a process for producing the above-mentioned phosphorus-containing carboxylic acid involves several problems as described hereunder. The phosphorus-containing carboxylic acid, especially a phosphorus-containing dicarboxylic acid has been produced, for instance, by allowing a phosphorus compound bearing a P—H bond such as 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to react with an unsaturated dicarboxylic acid such as fumaric acid, maleic acid or itaconic acid. The reaction is that called Michael addition reaction between the P—H bond in the phosphorus compound and C═C double bond in the unsaturated dicarboxylic acid. The reaction has been put into practice by mixing the phosphorus compound with the unsaturated dicarboxylic acid and heating the resultant mixture to a temperature in the range of 160 to 200° C., but has suffered the disadvantages of difficult temperature control due to the reaction being exothermic and also difficult separation of the phosphorus-containing dicarboxylic acid obtained after the reaction from the starting raw materials containing the phosphorus compound and the unsaturated dicarboxylic acid. Hence, in spite of being an extremely useful substance, the phosphorus-containing dicarboxylic acid is poor in productivity, is expensive in production cost and contains a large amount of the starting raw materials as impurities, whereby the practical application thereof is generally difficult.
In order to overcome the above-mentioned problems, there is disclosed a process for synthesizing the phosphorus-containing dicarboxylic acid at a reaction temperature in the range of 100 to 200° C. by the use of a lower saturated aliphatic monocarboxylic acid having 1 to 5 carbon atoms as a solvent {refer to Japanese Patent Application Laid-Open No. 176171/1996 (Hei 8)}. According to the above-mentioned production process, it is made possible to obtain a highly pure phosphorus-containing carboxylic acid in high yield by relatively removing generated heat upon the reaction of the phosphorus-containing carboxylic acid at a comparatively low temperature. However, the aforesaid lower saturated aliphatic monocarboxylic acid, which is a highly corrosive solvent having strong irritating-smell, is problematic in that perfect removal of the acid is difficult, thereby making it difficult to perfectly eliminating the irritating-smell even if cleaning is carried out repeatedly with great care by using a suitable solvent.
Moreover in the case where the carboxylic group in the phosphorus-containing dicarboxylic acid is used for the purpose of reacting with an other functional group, residual carboxylic group in the saturated aliphatic monocarboxylic acid reacts in the same manner as above, and for instance, when the carboxylic group is used as comonomer for polyester, there is a fear of causing the problem of unreasonably lowering the molecular weight of the objective polyester.
In such circumstances, there has eagerly been desired the development of a process for producing a highly pure phosphorus-containing carboxylic acid in high yield which process can facilitate industrial reaction control at a relatively low temperature and can also facilitate solvent removal.